For a designer wishing to use a
Linear Technology LT1072
switching regulator in the buck mode
and being forced to deal with high input
voltages, achieving the highest efficiency
possible poses a problem. If, for example,
you need to convert 20V to 5V at
a relatively low power level of 1.25W,
the quiescent current of the device itself
(typically 6 mA) will become an important
part of the circuit’s power
consumption.

Because the quiescent current is
relatively unaffected by the input voltage,
the power that the IC consumes is
directly proportional to its applied supply
voltage. If your system has an external
low-voltage supply available, you could run the IC from it—the LT1052
operates down to 2.6V. If such an auxiliary
supply is absent, you can operate
the IC from its own output by incorporating
a switch-over circuit (Figure 1).
Adding this feature boosts the supply’s
overall efficiency from 77% to 83%.

Figure 1 This switch-over circuit powers the regulator IC from the supply’s output after the supply starts up. Running the IC from the lower-voltage output instead of the input raises the supply’s efficiency from 77% to 83%.

When you first apply power to the
supply, the regulator has no output: R8
and D7 hold C6 discharged and the gate
of MOSFET Q4 at ground. Because Q4 is
turned off, the rising supply voltage pulls
the gate of Q3 up via R5. As the supply
voltage rises, Q3 turns on, applying the
full input voltage to the IC and allowing
the regulator to begin operation.

Once the regulator starts and the
output voltage rises, C6 begins charging through R8. When the voltage on the
gate of Q4 reaches about 2.5V, Q4 turns
on, pulling the gate of Q3 to ground and
shutting it off. This shutoff removes
the input voltage from the IC. As C5
discharges into the IC, D5 becomes forward-biased and supplies voltage from
the output to the IC.

If a power glitch or a momentary
short circuit causes the output voltage
to drop below the minimum that the
LT1072 needs to operate, diode D7 will
rapidly discharge C6, allowing the input
voltage again to be applied to the IC.
When the voltage rises again, normal
operation will resume.